The real estate tycoon says he's suing Bill Maher for a cool US$5 million, claiming the US comedian and political commentator has failed to pay him after Trump proved he's not - wait for it - descended from an orangutan.

It all kicked off last month when HBO host Maher, during a guest appearance on The Tonight Show with Jay Leno, quipped that he would donate $5m to a charity of Trump's choice if he could prove he is not the "spawn of his mother having sex with an orangutan."

Maher was mocking a similar offer Trump made to President Barack Obama during the presidential campaign season. Back in October, Trump said he would donate $5 million of his own money to charity if Obama would release his college records and passport information.

Check out video of Maher discussing his beef with Trump on The Tonight Show below:

Not one to shy away from a challenge, Trump responded by publicly sharing a copy of his birth certificate with Yahoo! News.

'NOT AN ORANGUTAN'

In a letter addressed to Maher, Trump's lawyer wrote: "Attached hereto is a copy of Mr. Trump's birth certificate, demonstrating that he is the son of Fred Trump, not an orangutan."

The letter demanded that Maher cough up the $5 million immediately, adding that Trump expected the cash to be donated to "the following five charities in equal amounts: Hurricane Sandy Victims, The Police Athletic League, The American Cancer Society, The March of Dimes, and The Dana-Farber Cancer Institute."

Trump then issued a warning to Maher.

"He made an absolute offer. I made an absolute acceptance. I showed him documentation, and he owes me $5 million, which I'm going to give to charities," Trump told US TV show Extra. "Let's see what happens, and if he doesn't give me the money, we'll probably sue him."

Nearly a month later, and after repeated letters asking for the dough, Trump appears to have had his fill of waiting.

According to scandal sheet TMZ, on Monday, Trump sued Maher for "breach of contract" in Los Angeles County Superior Court, claiming he has failed to live up to an "unconditional offer" to donate $5 million to charity.

Trump claims five "worthy charities" have been "deprived" of funds of millions of dollars, "which they have not been able to utilise for the benefit of their constituents".

The Celebrity Apprentice host dialled in to Fox News' TV show Fox & Friends yesterday, and announced his planned lawsuit.

"He promised me $5 million for charity if I provided certain information," Trump said over the phone to the hosts. "I provided the information. He didn't pay. So today I sue Bill Maher for $5 million for charity."

Trump also brushed aside the notion that Maher was joking.

"I don't think he was joking. He said it with venom. That was venom. That wasn't a joke. In fact, he was nervous when he said it; it was a pathetic delivery. But he said, 'I will give,' and said, 'I will accept.'"

Following his confab on the show, Trump tweeted: "@BillMaher didn't come through with his promised $5 million for charity so today I will sue him."

Not that Trump is that optimistic he'll prevail in court.

He added: "I don't know whether I will win or lose the @billmaher lawsuit but had an obligation to sue for charity."

"Bill Maher made an unconditional offer while offer while on The Jay Leno Show and I, without hesitation, accepted his offer and provided him with the appropriate documentation," said Trump in a statement, Politico reports.

"Prior demands for payment went ignored by Mr Maher despite the fact that the beneficiaries of this suit will ultimately be the charities... who would share equally the $5 million - something I am certain they can desperately use."

A spokesperson for Maher hasn't commented on the lawsuit. But the comedian himself had this to say on Twitter: